1. Field of the Invention
The present invention relates to a bone fixation device for immobilizing two or more bone pieces and keeping them firmly together whereby the pieces are immobilized at least for a period of time they need to undergo a natural welding, and more particularly the invention relates to blocking means for securing fastening means used in the device.
To the purpose of the present specification the term "bone piece" is meant any bone or bone piece resulting from the breaking of one or more bones, which pieces must be firmly immobilized together, for example to undergo natural welding, and the bone pieces, bones, or parts of these bones may be flat, long, short, or round bodies.
2. Description of the Prior Art
It is well known to use fixation plates to keep two or more bones or bone pieces together to promote natural and accurate welding of the bones. These plates are located against the bones to be welded and firmly fixed to the bone parts by fixation screws that are passed through fixation orifices generally located at the ends of the plates, and screwed in the bones. Thus, the plate remains firmly fixed to the bone parts to be joined and the parts are kept firm together in contact enough to facilitate correct welding of the bones. After some time, however, due to the movements of the patient, the screws may become loosen and may move out of the plate orifices with the serious consequences this would involve against the patient tissues, muscles or organs close to the plate.
Some approaches have been made to avoid as much as possible the loosing of the fastening screws to prevent the patient from being subject to the above mentioned consequences. A known technique consists of distorting or deforming a head of the fixation screw once the screw is entirely installed in the fixation plate and firmly screwed in bone. That is, once the screw has being passed through the plate orifice and secured into the bone tissue the head of the screw is distorted or deformed by punching or hammering the same within the plate orifice. The head deformation would prevent the head from moving relative to the orifice, whereby loosening of the screw is minimized. The technique, however, is very traumatic to the patient and risky to the integrity of the bone or bone pieces under treatment which are exposed to the important impacts from the hammering or punching operation.
Other fixation plates include an additional threaded bore parallel and adjacent the corresponding plate fixation orifice. A small locking screw is threaded in such bore, the small screw having a head and a threaded stem or shaft eccentrically pending from the head. The small locking screw is screwed without reaching the total screwing capacity of the screw and with a largest portion of the head, that is the portion more eccentrically extended relative to the stem, located away from contact with the head of the associated fixation screw. When the fixation screw has been firmly screwed in the bone, the locking screw is additionally screwed until the more eccentrically extended head portion abuts the head of the fixation screw and enters into an intimate and forced contact with the fixation screw to lock and retain the same in place by pressure against its head.
A fixation plate using locking means similar to the above disclosed also has an additional bore, adjacent and parallel to the fixation bore and an additional small locking screw with a head including a cut portion or straight sector. The cut portion of the screw head is located in such a way that the fixation orifice is not obstructed by this head and is free to receive the fixation screw therethrough. Once the fixation screw has been firmly fixed into the bone, the locking screw is rotated, by screwing the same in the fixation plate, so as the head of the locking screw out of the cut portion makes and intimate and forced contact against the periphery of the head of the fixation screw and lock and retain the same in position. In the two above embodiments provided in the prior art the same drawbacks remain without a solution being provided. That is, the problem of loosing the fixation screw seems to be solved by locking the fixation screw with an additional locking small screw which is now subject to the same loosing situation as it was with the fixation screw. In addition, there are many parts provided in the fixation plate, that is, the number of screws is duplicated as long as a new locking screw is necessary for locking each fixation screw, making the fixation plate more complex to be operated and highly risky for the patient.
Another fixation plate also known in the art consists of a plate including a flat disc with a central opening. Once the fixation screws have been screwed in position and the bone parts are firmly retained by the plate, the disc is located over the screw orifices, covering the fixation screw heads, and an additional screw is passed through the central opening of the disc and fixed against the plate by screwing the additional screw into the bone tissue. Again, while the number of additional locking means are reduced as compared to the above described fixation plates, this locking plate requires an additional screw which is not locked by any additional means to prevent this screw from loosening, with the enormous risk of having the disc released from the plate resulting in a dangerous potential situation for the patient.
Concluding, according to the prior art, all the fixation plates have been developed by approaching the problem of loosening fixation screws through the provision of additional screws in an attempt to lock the fixation screws in position and prevent the involuntary unscrewing thereof. Many additional elements are in this way involved in the approaches of the prior art, these many members not only increasing the risks for the patient but also increasing the work and time involved in installing of the plates and fixing of the same onto the bones.
It would be therefore very desirable and convenient to count on a new fixation plate capable of preventing the loosening of the fixation screws without using additional locking screws ore separate pieces subject to accidental releasing from the plate, within the patient's body, the new device being capable of being operated without involving too much time and work, as well as without complex operations being necessary.